You've tried to give him advice and he continues to ignore it. So, let the man swing and pay his taxes on his own. I do hope that the option you selected was to file separately and get money back.
You said it best...he fired you, so it's up to him to face the consequences of his actions, i.e., paying taxes. Let's face it, he's most likely relied on you for years and years to take care of things and he thought you would help him out of the tax situation...now he sees and feels (in the wallet) what it's going to be like now and in the future w/o your being there to rescue him and take care of things.
Leave him to stew in his pot of broth. He may or may not learn what to do about finances...right now...that's not your problem to solve for him.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
